German Offenlegungsschrift No. 3,520,139 describes, for example, a filter apparatus of this type. Basically, in these known filter apparatuses, the top wall is designed as a cover separable from the remaining housing part, specifically because, as a fixed component of the appliance, it remains connected unreleasably in operational terms to the feed and discharge lines. In order to clean the filter apparatus or exchange a filter cartridge, the cup-shaped housing part is unscrewed from the cover. For reasons of both reliable sealing and fastening, in practice, the cross-sections of the housing on filter apparatuses of the relevant generic order of size are exclusively circular. This has, above all, the following disadvantages:
(a) Opening the filter housing in or on the appliance during cleaning or the exchange of a filter cartridge often results in the escape of liquid; PA0 (b) In order to open the filter housing, a large amount of free space must surround it, in order, on the one hand, to gain access with a tool and, on the other hand, to make it possible to draw the cup-like housing part off from the cover. The circular cross-section, in any case, represents an unfavorable utilization of existing installation spaces; PA0 (c) It is not possible to produce multi-chamber filters because of the sealing problems which the separable housing presents; PA0 (d) Reverse-flow cleaning of the filter is possible only by changing over the direction of flow of the liquid in the appliance. PA0 (a) the top wall of the filter apparatus is resistant to bending, thicker than the adjoining peripheral wall and connected unreleasably thereto; and PA0 (b) the inlet and outlet connection pieces are equipped with coupling ends for easily releasable pressure-tight connection to the appliance feed and discharge lines respectively.